Planar r. f. load resistor for microstrip or stripline

ABSTRACT

A planar film RF load resistor useful, in some instances, from dc up into the microwave frequency range and, in other instances, particularly useful through broadband ranges in the microwave frequency spectrum range. It is a planar thin (or thick) film RF load resistor having a smaller radius edge in conductive contact with a like shaped radius edge of an RF signal thin (or thick) film conductor, and a larger radius outer edge in conductive contact with a highly conductive film plate. The radius centers are substantially the same for the planar film RF load resistors with both microstrip and stripline units.

United States Patent Ragan et a1.

[54] PLANAR R. F. LOAD RESISTOR FOR MICROSTRIP OR STRIP-LINE Lawrence H. Ragan, Dallas; Charles R. Pratt, Richardson, Tex.

Collins Radio Company, Dallas, Tex.

July 14, 1971 Inventors:

Int. Cl. ..I'I0lp l/26 Field of Search ..333/22, 81 A; 338/216, 306-309 [56] References Cited UNITED STATES PATENTS 3,585,533 6/1971 Denhard ..333/22 RF CONDUCTOR 151 3,678,417 [451 July 18, 1972 FOREIGN PATENTS OR APPLICATIONS 303,000 5/1959 Germany ..333/8l A Primary Examiner-Paul L. Gensler Attorney-Warren I-l. Kintnnger et a1.

[ ABSTRACT A planar film RF load resistor useful, in some instances, from dc up into the microwave frequency range and, in other instances, particularly useful through broadband ranges in the microwave frequency spectrum range. It is a planar thin (or thick) film RF load resistor having a smaller radius edge in conductive contact with a like shaped radius edge of an RF signal thin (or thick) film conductor, and a larger radius outer edge in conductive contact with a highly conductive film plate. The radius centers are substantially the same for the planar film RF load resistors with both microstrip and stripline units.

10 Claims, 8 Drawing Figures RESISTIVE FILM l6 METAL PLATE PATENTEU JUL 1 8 m2 SHEET 1 0F 2 RESISTIVE FILM CONDUCTOR l6 METAL PLATE FIG.3

FIG.4

PLANAR R. F. LOAD RESISTOR FOR MICROSTRIP OR STRIPLINE This invention relates in general to load resistors and, in particular, to a planar RF load resistor particularly useful in the microwave frequency range.

Reflectionless absorption of RF energy can be difficult to attain with, in transmission lines, many different methods being used to terminate a line in its characteristic impedance depending on the type of line. In waveguide, for example, a wedge shaped lossy material similar to that used in anechoic walls is commonly used. With coaxial transmission line an intervening disc of resistive material is in common usage, and with open wire line, used at lower frequencies, a single cylindrical resistor is generally adequate. In stripline a balanced double sided pill load is used but, heretofore, there have been few if any broadband, effective microwave load resistors for microstrip available. One approach when a broadband load was needed has been to construct a microstripto-coaxial line transition used with a coaxial line load. Expense is a factor with such transmission line terminations particularly with configurations having a low VSWR through their operational range of frequencies.

Microstrip ungrounded tapered resistors have been found to be somewhat restricted in useful frequency range while, grounded tapered resistors have higher VSWR than desired and also, an inherent hot spot. Simple grounded square resistors have too high a VSWR. Further, with tapered resistors one-quarter wavelength long at a pre-chosen center frequency the VSWR quickly goes up to unusable values with frequency variation to any material degree from the design center.

It is, therefore, a principal object of this invention to provide planar RF load resistors having low VSWR through a broad range in the microwave frequency range.

Another object is to provide planar transmission line planar RF load resistor terminations of even power distribution.

Afurther object is to provide such planar RF load resistor transmission line terminations that while highly reliable are relatively low cost units.

Features of the invention useful in accomplishing the above objects include, in planar RF load resistors for microstrip and stripline, a planar transmission line having a substantially semicircular termination end and a planar resist material section in conductive contact with substantially the entire peripheral edge of the transmission line semicircular termination end. The planar resist material section has an outer substantially semicircular edge having a radius center substantially common to the radius center of the transmission line semicircular end and with the resist material section outer semicircular edge in conductive contact with a planar conductive material plate that may be electrically connected to a ground plane plate.

Specific embodiments representing what are presently regarded as the best modes of carrying out the invention are illustrated in the accompanying drawings.

In the drawings:

FIG. 1 represents a portion of microstrip circuitry with a conductive film transmission line terminated by a planar RF load resistor in outer peripheral contact with a conductive plate;

FIG. 2, a partial side elevation view of the microstrip transmission line end termination of FIG. 1;

FIG. 3, a microstrip transmission line end termination quite similar to that of FIG. 1 with, however, additional resist material added for greater transmission line to load resistor contact;

FIG. 4, a differential element of a film resistor such as that of FIG. 1 useful in determining the resistance value for a film resistor of that geometry andwith an included angle greater or lesser than the 180 6 angle shown;

FIG. 5, a perspective view of such a transmission line resistor termination in stripline;

FIG. 6, a cutaway and sectioned side elevation view of the transmission line terminating resistor assembly of FIG. 7;

FIG. 7, a top plan view of a 50 watt load transmission line terminating resistor mounted in a case with the top removed; and,

FIG. 8, a VSWR to frequency in GHz diagram for the transmission line terminating resistor configuration of FIGS. 6 and 7.

Referring to the drawings:

The microstrip section 10 of FIGS. 1 and 2 is shown to include a thin film RF conductor 11 positioned on a relatively high dielectric constant ceramic material substrate 12. The thin film RF conductor 11 is shown to have a semicircular terminal end 13 with a radius r that is in conductive contact with a resistive film 14 having an interior radius peripheral edge with a radius r the same as the radius of the end 13 of the RF conductor 11 and an outer radius r peripheral edge 15 that is in conductive contact with a metal plate 16. Although the conductive metal plate 16 deposited on substrate I2 is shown to have an extension I7 around the edge of substrate 12 to conductive contact with conductive material ground plane plate 18 on the bottom of the substrate 12, as shown in FIG. 2, the conductive metal plate 16 could in some installations be a plate of a capacitor to in effect give an RF path to ground or other circuitry as may be required. Further, while the planar film RF load resistor 14 is shown as being semicircular with a 0 included angle of this included angle may be varied either to a greater 0 included angle or lesser angle in meeting various design objectives.

FIG. 3 is another embodiment very similar to the embodiment of FIG. 1 that includes a substantially semicircular resistor 14 with additional resistor material film added at each end to provide additional conductor 11' to resistor 14' conductive contact along sides of RF conductor 11 for a short distance from the semicircular end 13. It should be noted that any one of the elements, shown with any of the various transmission line end RF load resistor termination embodiments, or all of the planar components supported by a substrate may be constructed using either thin or thick film techniques.

With reference to the differential element showing of FIG. 4 and the resistance of a film resistor, consistent with geometry such as shown in FIG. 1, would be substantially in accord with the following:

The parameters of the problem are:

r radius of conductor end 13 r radius of film resistor edge 15 Oinc included angle of resistor p (sheet) resistivity of resistor r radius variable in polar coordinates 0 angle variable in polar coordinates R resistance to be found Define a differential resistance element as in FIG. 4.

Remembering the formula for resistance of a square element:

The resistance of the differential element is found to be: p (dr/rdB) Integrating over r from r to r we have the resistance of a differential wedge:

"2 dr L, p rd6 These differential wedges combine in parallel to form the resistance R.

Integrating conductance one obtains i p rdc Now p is uniform over the deposited area and'therefore not a function of r or 6. Likewise, r and 0 are independent variables.

in our particular case,.R= 50!), =100Q/ [3 r 12.5 mils then r 12.5.? 1750 100 62 mils These mathematical relationships are substantially the same with the embodiment of FIG. 3 with only slight variation in view of the extra planar resist material extension at both ends of the resistor 14. Obviously, variations induced with the extra resistive film material of the FIG. 3 embodiment from that of FIG. 1 would be of less consequence where ,the circuitry film elements are deposited on a high dielectric constant substrate such as a dense ceramic having a dielectric constant falling in the range of from, for example, 9 through 11 as opposed to a substrate like polyolefin having a much lower dielectric constant falling in the range of approximately 2.

Such planar RF load resistors are also quite applicable to stripline circuitry such as illustrated by FIG. 5. This includes a planar conductor 11" positioned between dielectric volumes l9 and 20 that in some instances would be air but generally speaking would be of like dielectric solid material chosen to meet particular design objective requirements. This is with planar semicircular resistor 14" interconnecting the semicircular end 13 and the semicircular r radius edge 21 of ground plane conductive plate 16" that is shown to be connected with outside ground plane conductive plates 22 and 23 by through interconnecting electrically conductive material pins 24.

Referring now to the microstrip embodiment of FIGS. 6 and 7 a rectangular section of microstrip 10 is similar in many respects to the microstrip section 10 of FIGS. 1 and 2 and has the same r and r and included angle relationships as with the embodiment of FIGS. 1 and 2. With this embodiment the thin film transmission line 11 extends from one edge of the rectangular ceramic substrate section 12 to the semicircle edge 13 end in conductive contact with the resistive film 14. The microstrip section is enclosed within a hermetically sealed conductive material connector box 25 having a lid 26 fastened to the top of the connector box 25 by conventional means, not shown, to enclose an internal rectangular opening and the microstrip section 10 positioned therein with the ground plane plate 18 side thereof in contact with opening bottom surface 27. A hermetically sealed coaxial threaded connector element 28 of conventional construction is threaded 29 into the connector case 25 with a center coaxial conductive rod 30 projecting through case opening 31 to RF signal conductive contact with thin film transmission line 11. After the connector 28 is completely threaded into position in the box housing 25 the coaxial rod lead 30 is soldered or braised to the thin (or thick) film transmission line 11 to insure a good RF connection between the rod 30 and the transmission line 11 before lid 26 is fastened in place on the box housing 25.

With a load resistor unit such as shown in FIGS. 6 and 7 designed for operation up through 1.1 GHZ,

where A wavelength in cm c velocity of propagation in a vacuum in CM/sec f frequency in Hz e relative detection of propagation medium so A/8= 1.025 cm so a resistor with r as large as 1.0 cm should work well to about 1.1 GHz. (r and the larger resistor was 0.356 inches or about 0.91 cm.)

A planar RF conductor 11 load resistor 14 microstrip section 10' unit in accord with FIGS. 6 and 7 with a measured dc resistance equal to 49.693 ohms has a VSWR to 61-12 characteristic from the center conductor input of coaxial connector unit 25 through the load resistor 14 as shown in FIG. 8.

Whereas this invention is here illustrated and described with respect to several specific embodiments hereof, it should be realized that various changes may be made without departing from the essential contributions to the art made by the teachings hereof.

We claim:

1. In an RF load resistor circuit unit: a planar resistor; a planar RF signal line substantially coplanar with said planar resistor; a planar conductive material plate substantially .coplanar with said planar resistor; said planar RF signal line being formed with substantially a semicircular edged termination end; said planar resistor having an inner radius edge conforming to the semicircular edged termination end of said planar RF signal line and in electrical conductive contact with a substantial portion of said semicircular edged termination end; said planar resistor also having an outer arced substantially constant radius edge with a radius center substantially common to the radius center of the planar RF signal line semicircular edged termination end; and said planar conductive material plate being formed with an inner substantiallyv constant radius edge conforming to the outer radius edge of said planar resistor and in electrical conductive contact with a substantial portion of said outer radius edge of said planar resistor; and structural means supporting said planar resistor, said RF signal line, and said planar conductive material plate in coplanar interconnected relation.

2. The RF load resistor circuit unit of claim 1, wherein the inner radius edge of said planar resistor is in electrical conductive contact with substantially the entire extent of the semicircular edged termination end of said planar RF signal line.

3. The RF load resistor circuit unit of claim 2, wherein the outer radius edge of said planar resistor is in electrical 'conductive contact with said conductive material plate through substantially an included angle of 4. The RF load resistor circuit unit of claim 3, wherein said planar resistor is substantially a semicircular resistor from edge to edge.

5. The RF load resistor circuit unit of claim 2, wherein the planar resistor is formed with end extensions in conductive contact with side edge portions of said planar RF signal line adjacent said semi-circular edged termination end.

6. The RF load resistor circuit unit of claim 1, wherein said unit is a microstrip unit.

7. The RF load resistor circuit unit of claim 6, wherein said planar conductive material plate is connected to a ground plane plate; and with said microstrip unit including a solid dielectric sheet having two sides with said RF signal line, planar resistor and planar conductive material plate on one side, and said ground plane plate on the other side; andconductive means interconnecting said planar conductive material plate and said ground plane plate.

8. The RF load resistor circuit unit of claim 7, wherein said microstrip unit is enclosed in a conductive material box having a coaxial connector providing through wall RF signal connecopposite outside ground plane plates are provided on the outside of two dielectric material sheets sandwich enclosing said coplanar circuit elements in stripline relation; and conductive interconnect means interconnecting said planar conductive material plate and the two opposite outside ground plane plates. 

1. In an RF load resistor circuit unit: a planar resistor; a planar RF signal line substantially coplanar with said planar resistor; a planar conductive material plate substantially coplanar with said planar resistor; said planar RF signal line being formed with substantially a semicircular edged termination end; said planar resistor having an inner radius edge conforming to the semicircular edged termination end of said planar RF signal line and in electrical conductive contact with a substantial portion of said semicircular edged termination end; said planar resistor also having an outer arced substantially constant radius edge with a radius center substantially common to the radius center of the planar RF signal line semicircular edged termination end; and said planar conductive material plate being formed with an inner substantially constant radius edge conforming to the outer radius edge of said planar resistor and in electrical conductive contact with a substantial portion of said outer radius edge of said planar resistor; and structural means supporting said planar resistor, said RF signal line, and said planar conductive material plate in coplanar interconnected relation.
 2. The RF load resistor circuit unit of claim 1, wherein the inner radius edge of said planar resistor is in electrical conductive contact with substantially the entire extent of the semicircular edged termination end of said planar RF signal line.
 3. The RF load resistor circuit unit of claim 2, wherein the outer radius edge of said planar resistor is in electrical conductive contact with said conductive material plate through substantially an included angle of 180* .
 4. The RF load resistor circuit unit of claim 3, wherein said planar resistor is substantially a semicircular resistor from edge to edge.
 5. The RF load resistor circuit unit of claim 2, wherein the planar resistor is formed with end extensions in conductive contact with side edge portions of said planar RF signal line adjacent said semi-circular edged termination end.
 6. The RF load resistor circuit unit of claim 1, wherein said unit is a microstrip unit.
 7. The RF load resistor circuit unit of claim 6, wherein said planar conductive material plate is connected to a ground plane plate; and with said microstrip unit including a solid dielectric sheet having two sides with said RF signal line, planar resistor and planar conductive material plate on one side, and said ground plane plate on the other side; and conductive means interconnecting said planar conductive material plate and said ground plane plate.
 8. The RF load resistor circuit unit of claim 7, wherein said microstrip unit is enclosed in a conductive material box having a coaxial connector providing through wall RF signal connection from the exterior to the interior of said box with a coaxial connector center conductor RF signal path connected to said planar RF signal line.
 9. The RF load resistor circuit unit of claim 1, wherein said unit is a stripline unit.
 10. The RF load resistor circuit unit of claim 9, wherein two opposite outside ground plane plates are provided on the outside of two dielectric material sheets sandwich enclosing said coplanar circuit elements in stripline rElation; and conductive interconnect means interconnecting said planar conductive material plate and the two opposite outside ground plane plates. 